Client NameLocation: Project TitleDate
Corrosion, Erosion, and Wetted PartsA Heavy Metal Discussion
By Eric Lofland
Scope of This Presentation
• Explain some of the basic features of steels
• Define the principle problems in material selection
• Provide historical examples and mechanisms for these
problems
• Define and summarize the basis of NACE MR0103 and
MR0175 codes
• Offer some advice for how to tackle challenging
applications
What Is A Metal, Really?
What Is A Metal, Really?
• Generally a crystalline solid
at room temperature
• Exhibits metallic bonding
• High melting point
• Conduct electricity and heat
• Great material for a
chemical process
Some Basic Crystalline Structures
• Structures form a
lattice
• That lattice strongly
influences the
physical properties of
a metal
• Can be viewed like a
physical structure
Phase Diagram of Iron
Ferrite
• α-phase Iron
• Body-centered cubic
structure
• Ferromagnetic
• Does not dissolve
much carbon due to
lack of space in the
lattice
Austenite
• γ-phase Iron
• Face-centered cubic
structure
• Not magnetic
• Dissolves more carbon
due to more lattice space
Martensite
• Formed by rapid quenching
of austenite
• Body-centered tetragonal
strucure
• Magnetic
• Needle-like microstructure
• Harder, but more brittle
Austenite vs. Martensite
Austenite Martensite
What Is Steel?
• Alloy consisting primarily of iron
• Other metals added for various properties
• Carbon steel – primarily iron and carbon
• Stainless steel – chromium added for corrosion
resistance, forms a passive layer of chromium oxide
• High strength, relatively low cost
A Basic Guide to Stainless Steel Alloys
• Carbon adds structural
strength
• Chromium adds corrosion
resistance
• Nickel stabilizes the austenite
phase
• 200 and 300 series –
Austenitic
• 400 series – Martensitic and
Ferritic
SAE
designationType
1xxx Carbon steels
2xxx Nickel steels
3xxx Nickel-chromium steels
4xxx Molybdenum steels
5xxx Chromium steels
6xxx Chromium-vanadium steels
7xxx Tungsten steels
8xxxNickel-chromium-
molybdenum steels
9xxx Silicon-manganese steels
(Jeffus 635)
What Causes An Installation to Fail?
What Causes An Installation to Fail?
• Excess temperature or pressure
• Physical property of selected material
• Outside the scope of this presentation
• Erosion
• Material is subject to excessive wear and tear
• Corrosion
• Material is not chemically compatible service
Erosion
• The gradual destruction of a
material due to physical stress
• Opposed to corrosion, which is
caused by chemical stress
• Physical stresses include
• Hydrodynamic stress
• Solid particulates
• Flashing and cavitation
• Solutions are based on physical
properties of materials
Erosion by Particulate
• Caused by particle impacts with a surface
• Dependent on particle properties,
velocity, angle, and frequency of impact
• Most predictive equations for damage are
empirical
• Of particular concern for elements in the
flow path and elbows in pipe
• Of particular interest for the oil and gas
industry
Erosion by Particulate – The Mechanism
Brittle Mechanism
Erosion by Particulate – Kinetic Energy
• Damage caused by particles is directly related to kinetic
energy
• Most empirical models incorporate mass and velocity as
important factors
𝐸𝐾 =1
2𝑚𝑣2
𝐸𝐾 = Kinetic energy of impact
𝑚= Mass of particle
𝑣= Velocity of particle
Erosion by Particulate – Other Factors
• Frequency and duration of exposure
• What is the solids content?
• How often does exposure occur?
• Angle of impact
• Brittle objects struck directly will sustain more
damage
• Relative Hardness
• The higher the hardness of the particle as compared
to the target, the greater the damage
Erosion by Particulate – What Does It All Mean?
• Many proposed equations predicting erosion rate from
the previous factors
• For choosing a material, exact rate of loss is difficult to
predict and less useful than a qualitative assessment
• Consider the following order of importance when
assessing risk:
Velocity > Relative Hardness >> Particle Size =
Solids % > Angle of Impact
Most Important: Velocity
• Paramount importance
• Most equations raise velocity
to an exponent
• Liquid streams have lower
velocities, usually lower risk
Velocity > Relative Hardness >> Particle Size =
Solids % > Angle of Impact
Very Important: Hardness
• Is the particulate hard enough
to cause damage?
• Globules in hydrocarbon
streams are usually not
considered.
• Sand on the other hand…
Velocity > Relative Hardness >> Particle Size =
Solids % > Angle of Impact
Less Important: Size, Solids %, and Angle
• Particle Size
• Larger particles have low velocity
• Solids %
• More useful for trying to estimate
“when” than “if”
• Angle of Impact
• Occasionally useful to assess
where the particle is going
Velocity > Relative Hardness >> Particle Size =
Solids % > Angle of Impact
Erosion by Flashing and Cavitation
• Flashing and Cavitation
occur when a liquid
changes phase due to
pressure drop
• Both phenomena greatly
increase the physical stress
on wetted parts
• Liquids near boiling point or
at areas of heavy pressure
drop are at the greatest risk
Erosion by Flashing and Cavitation
• Volume of a vapor at STP is
about 3 orders of
magnitude greater than
liquid
• An in-depth explanation of
these phenomena is
outside the scope of this
presentation
Signs You Are Facing Erosion
• High velocity stream with
solid particulate
• Hard solid particulates in
stream
• Liquid stream near boiling
point
• Liquids stream with high
pressure drop
Industry Solutions to Erosion
• Step 1: Can the source of wear be mitigated or removed
completely?
• Step 2: Consider a hardened alloy to extend life of
wetted parts.
• Step 3: Verify selected material against existing similar
installations if possible.
• Step 4: Verify that the selected material is chemically
compatible with the process fluid.
What Alloys to Use in Erosive Services
• Martensitic steels (400 Series) may be acceptable for
less rigorous installations.
• Precipitation-hardened steels such as 17-4PH are also
acceptable for slightly more rigorous installations.
• For highly rigorous applications, consider hardfacing an
element with Stellite 6 or other chromium-cobalt alloys.
• In extreme cases, an entire element can be made out of
Stellite 6.
Corrosion
• The gradual destruction of a
material due to chemical attack
• Opposed to erosion, which is
caused by physical stress
• Chemical attacks can occur on
multiple vectors
• Solutions are based on chemical
properties of materials on a
case-by-case basis
Corrosion – The Math
• Corrosion is a chemical reaction
• Common chemical reaction model
For chemical A in reaction ,𝐴 + 𝐵 → 𝐶 + 𝐷
−𝑟𝐴 = 𝐴𝑒−𝐸𝑎𝑅𝑇 𝐶𝐴𝐶𝐵
Corrosion – The Math
Corrosion – The Math
For chemical A in reaction ,𝐴 + 𝐵 → 𝐶 + 𝐷
−𝑟𝐴 = 𝐴𝑒−𝐸𝑎𝑅𝑇 𝐶𝐴𝐶𝐵
−𝑟𝐴 = Rate of disappearance of A (Corrosion)
𝐴= Prefactor (Constant)
𝐸𝑎= Activation Energy (Constant)
𝑅= Universal gas constant
𝑇= Temperature
𝐶𝐴= Concentration of A
𝐶𝐵= Concentration of B
Common Vectors for Corrosion
• Acid/Base Reactions
• Hydrogen Embrittlement
• Sulfide Stress Cracking
• Stress Corrosion Cracking
Problem #1 Acids and Bases
• Acids and bases attack metals
via different mechanisms to
form ionized salts
• Strongly influenced by
temperature and concentration
of acid/base
• Charts are available for
chemical compatibility of
common alloys with various
chemicals
Possible Metallurgy Solutions
• For low concentrations of corrosives, austenitic (300
Series) stainless steels can work (Iron-Chromium-Nickel).
• For higher concentrations, more exotic compounds are
required.
• Super-Austenites (Iron-Extra Chromium-Extra Nickel-
Molybdenum-Nitrogen)
• Hastelloy C (Nickel-Molybdenum-Chromium)
• Monel (Copper-Nickel)
Problem #2 Hydrogen Embrittlement
• Hydrogen atoms diffuse into
the surface of a metal
• Hydrogen atoms recombine
to form H2 bubbles in the
metallic matrix
• Bubbles in the metallic
matrix greatly embrittle the
metal, which leads to failure
under normal operating
conditions
Assessing Risk and Determining the Solution
• Any metal exposed to hydrogen, particularly at elevated
temperatures, is susceptible
• Harder metals are more susceptible to embrittlement
• Common solutions include prevention and heat
treatment to remove hydrogen
Problem #3 Sulfide Stress Cracking
• H2S causes embrittlement and
cracking of metals
• Causes sudden catastrophic
failure
• Particularly important in
oil/refining applications, due to
the high quantities of H2S
• Complex mechanism
extensively studied by NACE
What is NACE?
• NACE International was established in 1943
• Formerly known as the National Association of
Corrosion Engineers
• Professional organization that publishes test methods,
standard practices, and standards for material selection
• Review and revise the perennial standards to prevent
Sulfide Stress Cracking, NACE MR0103 and MR0175
NACE MR0103 vs. NACE MR0175
• NACE MR0175 was created for
upstream (oil and gas production)
environments
• Generally more rigorous than
downstream
• Higher chloride ion concentration
• Lower pH
• NACE MR0103 was created for
downstream (refining) environments.
• Generally less rigorous
NACE: Important Notes
• Read NACE Safely!
• Neither standard makes an
effort to rank materials based
on SSC resistance.
• NACE does not suggest
materials to use.
• Both standards are living
documents and can be added
to.
Sulfide Stress Cracking - The Mechanism
• Metals react with H2S in process
fluid to release atomic hydrogen
• Atomic hydrogen accumulates in
the metal matrix
• Reaction is cathodic (electrons
are donated to metals)
• Tensile stresses in the metal form
cracks
Sulfide Stress Cracking - The Mechanism
The Environment – What Factors into SSC?
• Concentration of H2S in aqueous
or gaseous phase
• Temperature
• Substances are “charged” with
hydrogen at high temperatures
• Failure occurs most frequently
at ambient temperatures
• pH and Chloride Ion Concentration
• Extreme pH in either direction
• Chloride ions accelerate SSC
Residual Stress and PWHT
• Welds are a focal point of SSC
• When a material is welded, the
area is heated unevenly
• Variable tensile forces develop
due to temperature differences
• Post Weld Heat Treatment
relieves the stress
How Hard Could It Be?
• NACE provides hardness
limits for alloys
• Hardness is ameliorated
by temperature change
• NACE provides acceptable
procedures
• These often include
moving between metallic
phases
How Does This Affect My Installation?
• Austenitic steels tend to have less stringent hardness
requirements
• Welds are of particular concern – PWHT often required
The NACE Takeaway
• NACE is not so much a metal selection guide as it is a
set of practices
• A good place to start is to use existing installations to
choose an alloy
• Use NACE to identify vulnerabilities and as a guide to
make the alloy work, making changes as required
• Vendors of instruments often have NACE certificates for
instruments
Problem #4 Stress Corrosion Cracking
• Family of reactions that proceeds via
a different mechanism from Sulfide
Stress Cracking
• Does NOT affect the finish of the
metal
• Can occur at low reactant
concentrations
• Commonly seen in chloride solutions
with austenitic steels and ammonia
solutions with copper alloys
Historical Example – Season Cracking
• British forces in India were forced
to spend a lot of time inactive
during monsoon season.
• Ammunitions were stored in
barns.
• It was found that brass cartridges
would spontaneously crack.
• It was discovered in 1921 that
this was caused by ammonia
from horse urine in the barns.
Stress Corrosion Cracking – The Mechanism
• Annodic reactions occur in
irregularities of metal surface
• Metal is oxidized to a positive
ion, which is dissolved in water
• Reaction site forms ions that
attract ionic reactants
• Attracted ions concentrate at
the reaction site and make
things worse
Possible Metallurgy Solutions
• Use a metal that is chemically
compatible
• For season cracking, use a non-
copper alloy if possible or the
anneal the metal
• For chlorides, consider a duplex
steel (part austenite, part ferrite)
• In extreme cases, exotic alloys
such as Hastelloy or titanium
alloys can be used
The Moral of The Story
• Consider all possible scenarios when choosing
materials for your process.
• Try eliminating or mitigating an erosive service first. If
this fails, harden the materials.
• Choose materials that are chemically compatible with
your process under ALL possible conditions.
• Develop a communicative relationship with your process
engineer.
Work Cited
• A comprehensive review of solid particle erosion modeling for oil
and gas wells and pipelines applications, Parsi et al, Journal of
Natural Gas Science and Engineering, Volume 21, Pg 850-873.
• Chloride stress corrosion cracking in austenitic stainless steel,
Parrot and Pitts, Harpur Hill, 2011.
• NACE MR0103-2012, Materials Resistant to Sulfide Stress
Cracking in Corrosive Petroleum Refining Environments, NACE
International, 2012.
• NACE MR0175-2015, Petroleum and natural gas industries—
Materials for use in H2S-containing environments in oil and gas
production, NACE International, 2015.
Questions?